Typical modern, domestic fluid-fired furnaces are designed and operated with flue gas smoke as the only pollutant emission consideration. Field studies on No. 2 fuel oil fired furnaces have revealed that existing furnaces of conventional design produce a relatively high average of approximately 1.8 grams NO.sub.x (as nitric oxide) per kilogram of fuel oil burned when tuned to the industry accepted operating conditions producing less than No. 1 Bacharach smoke. These furnaces typically have uncooled refractory-lined fireboxes that maintain high combustion temperatures which, in turn, aid in the complete combustion of carbonaceous compounds, thus avoiding the excessive production of carbonaceous pollutants (e.g., smoke and carbon monoxide).
High flame zone temperatures also help to stabilize combustion and prevent excessive combustion-generated noise. This uncooled type of firebox design, i.e., higher combustion zone temperature, is a major influence upon the resulting high oxides of nitrogen emissions from existing furnaces. To reduce heat losses out the flue, i.e., increase thermal efficiency, burner operation is targeted for minimum excess air, which has led to the introduction of flame retention burners. However, higher adiabatic flame temperatures result from lower excess air conditions, and this, coupled with the increased combustion gas residence time induced by the intense recirculation and mixing characteristic of the typical flame retention devices, promotes the formation of even higher flue gas concentrations of oxides of nitrogen.
When an oil burner is being fired, exhausted product gases carry off substantial sensible heat and the heat of vaporization of combustion generated water vapor. This convection of heat up the flue is the greatest source of thermal inefficiency in residential space heating. Flue gas sensible heat losses may be reduced by lowering excess air in the burner or by lowering flue gas temperatures or both. Steady state thermal efficiencies could be increased by 5 to 10% simply by reducing the flow of air required by the burner. However, if the air flow is reduced to less than some nominal value (typically 20 to 50% for conventional furnaces), furnace operation is constrained by carbonaceous pollutant formation. Similarly, steady-state efficiency could be increased by 10 to 15% by lowering flue exhaust temperature. This approach, in conventional residential heating systems, is constrained considerably by the reliance on the stack effect to provide an adequate draft in the firebox and by corrosion problems if combustion-generated moisture and acids are allowed to condense in the furnace, flue or chimney.
Typically, heated and humidified living-space air is used for burner combustion air and for air that is drawn into the furnace flue through a barometric draft control device. The use of living-space air for these purposes contributes to overall residence thermal inefficiency, since the conditioned air so used is replaced by infiltrating ambient outdoor air into the living-space.
When a burner is not being fired, the stack effect induces a natural draft flow of air through the burner, firebox, heat exchanger and flue. Warm furnace components are cooled by this flow and, again, heat is lost up the flue. This loss can reduce net thermal efficiency by as much as 5% and is perhaps, the largest of several transient heat losses which cause cycle averaged efficiencies to be lower than steady state values.
The flue gases of a residential oil furnace should conform approximately with the following air pollutant emission criteria:
carbon monoxide emissions, no greater than 1.0 g/kg of fuel burned, PA1 unburned hydrocarbons, no greater than 0.1 g/kg of fuel burned, PA1 nitrogen oxides, no greater than 0.5 g (as NO)/kg of fuel burned, PA1 smoke, no greater than Bacharach no. 1.
Achieving the pollutant criteria involves: optimizing combustion zone conditions; eliminating burner start spike emissions of carbon monoxide, smoke and unburned hydrocarbons; and reducing substantially the steady-state NO.sub.x emission level.
It is, accordingly, an object of the present invention to provide a new and improved fluid fired domestic furnace that approximately meets the stated pollution criteria and is highly efficient.
A further object of the invention is to provide a new and improved oil fired, domestic furnace that is more efficient and less polluting than existing furnaces, and which does not require substantial departures from design and operating principles of existing furnaces.
Another object of the invention is to provide a new and improved oil fired, domestic furnace that uses a relatively small amount of fuel.
An additional object is to provide a new and improved oil fired, domestic furnace having minimum start spikes.